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Physical & Chemical properties

Dissociation constant

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Description of key information

A study conducted on boric acid according to OECD guideline 112 gave a dissociation constant for boric acid of 8.94 at 20 ºC. The dissociation constant for boric oxide as such cannot be determined because boric oxide is converted into boric acid/borate upon dissolution in water:
B2O3 + 3H2O = 2B(OH)3. The dissociation constant found will be the dissociation constant for boric acid.

Key value for chemical safety assessment

pKa at 20°C:
8.94

Additional information

A study conducted on boric acid according to OECD guideline 112 gave a dissociation constant for boric acid of 8.94 at 20 ºC.

 

The dissociation constant for boric oxide as such cannot be determined because boric oxide is converted into boric acid/borate upon dissolution in water:

B2O3+ 3H2O = 2B(OH)3. Thedissociation constant found will be the dissociation constant for boric acid.

 

At low boron concentrations (B ≤ 0.025 M) the following equilibrium is found:

B(OH)3+ 2H2O ↔ B(OH)4-+ H3O+with pKa = 9.0 at 25 °C.

 

In dilute aqueous solutions (B ≤ 0.025 M) boric acid exists as undissociated boric acid B OH)3at pH < 7, at pH > 11 the metaborate ion becomes the main species in solution. In between values (pH 7 – 11) both species are present.

 

At higher boron concentrations (B > 0.025 M) an equilibrium is formed between B(OH)3, polynuclear complexes of B3O3(OH)4-, B4O5(OH)42-, B3O3(OH)52-, B5O6(OH)4-and B(OH)4-. In short B(OH)3↔ polynuclear anions ↔ B(OH)4-.

 

In acid solutions at pH < 5, boron is mainly present as B(OH)3and in alkaline solutions at pH > 12.5 boron is mainly present as B(OH)4-. At in between values (pH 5 – 12) polynuclear anions are found as well as B(OH)3and B(OH)4-.

 

The dissociation constant depends upon temperature, ionic strength and presence of group I metal ions (Na, K, Cs).

 

In the presence of metal ions (e.g. Na, Mg, Ca) ion-pair complexes are formed, which further reduce the undissociated boric acid concentration:

Mn++ B(OH)4-↔ MB(OH)4(n-1)+

 

These ion pair complexes are expected to be present in solutions of disodium tetraborate, disodium octaborate and buffered solutions of boric acid and boric oxide.